Recent nanotechnology is given attention regarding nanophase material since the nanophase material has a property better than a conventional material. In the nanotechnology, manufacturing, generation, transportation, deposition, and measurement of nanoparticles are essential. Charging particles or obtaining an equilibrium charged state is an effective method for controlling the nanoparticles in transportation or deposition processes. For example, the nanotechnology is used in crystal film formation by electrostatic deposition of charged colloid nanoparticles, and synthesis of two component system nanoparticles by attachment of nanosize aerosols each charged to opposite polarities. Charging of nanoparticles is indispensable in measurement of nanoparticles using static electricity such as, nanocluster DMA and particle beam mass analyzer.
Particles are normally charged as a result of impact between the gas ion and the particles. The charging state of the particles can be divided into unipolar charge and bipolar charge in accordance with the charged state. A bipolar charging device using radiation is usually used. A radiation source includes radioactive substances such as americium (241Am), krypton (85Kr), polonium (210Po) and the like.
FIG. 1 is a cross sectional view showing one example of a conventional bipolar charging device using radiation. In FIG. 1, an inlet duct 2 for introducing aerosols, and a outlet duct 3 for exhausting the aerosols are provided on both ends of a cylindrical chamber 1. A radiation source 4 such as americium (241Am) is arranged at an intermediate part of the chamber 1. Rectifying plates 5, 6 for rectifying the aerosols passing through the chamber are arranged on the left and the right. The rectifying plates 5, 6 include a plurality of fine openings, are used for rectifying the aerosols, and are arranged in the vicinity of the inlet duct 2 and the outlet duct 3, as shown in the figure. By arranging the radiation source 4 within the chamber 1 and introducing the aerosols into the chamber 1, the fine particles of the aerosols are charged by a large amount of positive and negative ions, and the equilibrium charging state can be obtained when the average charged amount is substantially zero.
Further, a charging device for generating unipolar charged ions is recently given attention for its wide range of applications. The conventional unipolar charging device, as shown in FIG. 2, includes a chamber 11 configured with a cylindrical part 12 made of resin for side surfaces, and with electrodes 13, 14 for upper and lower surfaces thereof. Voltage is applied between the upper and lower electrodes 13, 14 from a high voltage power source 15, and an ammeter 16 for measuring minute current is connected therebetween. A radiation source 17 of americium (241Am) is arranged on the electrode 14 at the lower surface of the chamber 11. If the height of the chamber is for example, 90 mm, α ray only reaches to about 40 mm due to its range; thus, bipolar ions are generated at the lower part of the chamber. As an electric field is generated, ions of desired polarity move towards the upper part of the chamber 1. Therefore, when aerosols are flowed through the inlet duct 18, the unipolar particles are discharged from the outlet duct 19, thereby achieving unipolar charging.
The charging device using corona discharge is capable of generating unipolar or bipolar high concentration ions, and is thus widely used. According to this method, when direct current or alternating current voltage of high voltage is applied to the electrode, unipolar or bipolar ions can be generated in the vicinity of the electrode.
However, in the conventional device for charging the aerosol particles using radiation, the half-life of the radioactive substance is long and thus has a problem in terms of safety. For example, americium requires 432.2 years, and krypton (85Kr) requires 10.72 years. Thus, management over a long period time is difficult. Further, polonium (210Po) has a short half-life of 138 days, and thus has a problem in that the line source must be changed every few months.
Further, the conventional unipolar charging device using radiation has small generation number of ions, and has losses inside the charging device or inside a piping, and thus has a disadvantage of being difficult to use unipolar charged nanoparticles for various applications. It also has a disadvantage in that a charging operation can not be stopped when necessary.
Additionally, the charging device using corona discharge generates ozone, causes corrosion of electrodes during discharge, and generates particulate substances by the gas phase reaction at a strong electrical magnetic field, and thus has a disadvantage of polluting air. The corona discharge also has a disadvantage of generating current noise.
The present invention aims to provide a charging device of aerosol particles that is safe and easy to handle in place of the conventional charging device using radiation source or corona discharge.